The immune response is a highly regulated process with deficient activation being manifested by decreased resistance to infections and probably malignancy and altered, poorly controlled or excessive activation by allergy and autoimmunity. Triggering and modulation of the inflammatory and/or immune response appear to be exerted through specific cell surface receptors for antigens, immunoglobulins, complement and other factors. These receptors on immunocompetent cells are being investigated in regard to their susceptibility to pharamacologic manipulation, mode of biochemical linkage to the interior of the cell, and physicochemical structure. Utilizing affinity chromatography and immunoprecipitation, isolation and Physicochemical characterization of the Fc and C3b receptor on immunocompetent cells is being pursued. Phagocytosable particles, sensitized with immunoglobulins amd/or complement, are exposed to granulocytes and monocytes (macrophages) so as to engage the receptor under study. In parallel studies these same cells are challenged with chemotactic factors and other soluble activators. The alteration of cellular metabolism consequent to the attachment and/or ingestion of the particles and chemotactic factors is then examined in regards to cyclic nucleotide levels, protein phosphorylation, and arachidonic acid metabolites. The key objective of these investigations is to obtain a better understanding of the mechanism whereby surface phenomena, consisting of the attachment and in some cases ingestion of immune-complexes and the binding of soluble activating factors to specific receptors occur and then mediate and/or modulate a cellular response.